Depends, @katewhinesalot. To give an example, when driving through Dartmoor (I was the passenger) and seeing the landscape and walls/sites of ancient settlements, I can superimpose people, crops, animals, different weather and buildings over the top, so the visualisation is working with what I actually see.
If I need to visualise something completely different, I sort of 'switch off' my sight - I stop focusing on anything in particular and it's all still 'there' but I'm paying little attention to it - I'll notice movements immediately, though, such as a crow just flying from right to left in line with the top of my front window as I thought about what I'd type. I don't think 'there's a crow', I've seen it and know what it is without having to make the words in my head.
If my eyes are closed, it can become a more detailed image - I can place myself in it more easily, know what's behind me or somewhere in the area, think of how the breeze would feel on my arm and the sound/look of the tree as its leaves are moved. Or I get distracted by a part of the image that I didn't know existed - I have issues with attention span - and go off on a complete tangent.
For example, the crap cut price CBT counselling through the GP 'meditation' consisted of sitting in a noisy room facing onto a noisy street and thinking of walking down some steps. Great. Except I was picturing the idiot revving his car up outside, the doctor calling the patient's name out in the waiting room, thinking the blind in the room needed fixing because it was squeaking, wondering whether the air con was ever going to be switched on - AND getting distracted on the imaginary steps because I'd be far more interested in watching the birds in the trees and wandering off into the countryside than walking down into some dark, enclosed courtyard with a bunch of tatty plants in pots.